In the process of exploring for hydrocarbons, there are many types of formations that are known for trapping hydrocarbons. It is not uncommon that formations for trapping hydrocarbons are beneath complex geologic structures. Complex geologic structures may only be complex in that it is difficult to resolve the complex structure using seismic imaging or difficult to resolve what is beneath the complex structure using seismic surveying, or both. A simple structure that is complex from a seismic imaging standpoint is one that has gas in it. Gas attenuates compressive waves and, to the extent any compression waves are reflected back to the surface, they are noisy and essentially undecipherable. Big gas deposits may actually create seismic “black holes” hiding or obscuring large areas that could hold significant hydrocarbon deposits.
Salt domes, especially where there are large overhangs, are an example of other relatively simple structures that create complex seismic images. The overhangs caused by iso-static forces at the sides of salt domes tend to be very interesting to hydrocarbon producers because the up-thrust of a salt dome tends to seal the adjacent formations that are lifted above their surrounding plane. Substantial amounts of hydrocarbons may end up trapped at the interface of a salt dome and a hydrocarbon bearing formation, especially where the interface is under the overhang of a mushroom-shaped salt dome. The overhang may take the shape of a mushroom or an anvil or a curling wave in the ocean. Since compression waves travel relatively slow through rock, but comparatively fast through salt, seismic energy that has passed through a salt formation, especially a salt formation with an irregular or complicated shape appears scattered in ways that are not easily resolvable into a coherent image.
Ray tracing techniques have been tried, unfortunately simple ray tracing techniques cannot adequately image the data without severe errors in propagation. Techniques based on one-way wave equation (usually called wave equation in convention) (e.g., Xu, S. and Jin, S., 2005) have inherent problem/limitation in amplitude/energy accuracy and in wide-angle travel time accuracy of the wave propagation especially in models with sharp velocity contrast, e.g. the sediment-salt interface.
Conventional techniques of survey modeling and survey design are not particularly well suited for complex geological situations or for the problems mentioned above. Conventional techniques are primarily ray trace modeling that envisions the seismic energy assuming a straight line path through the earth and reflecting to the seismic receivers. The problem is that the ray trace methods assume that the rays are traveling through homogenous media and reflect or refract at the bed interfaces. Ray trace modeling follows direct lines and does not handle compaction curves cleanly. Ray tracing also does not handle anisotropy well and does not allow accurate representation of dispersion. The advantage of ray trace methods is that they are relatively simple and may provide a coherent image of complex geologic structures. However, full wave equation modeling is far more accurate in its portrayal of seismic energy, and could provide far more accurate images, but computer resources and coding has heretofore been unavailable for reasonably sized acquisition or imaging modeling projects.